- Lemons are fruits which are sour.
- A lemon is a fruit which is sour.
Are these two sentences interchangeable? To me the first sentence looks awkward. If there is any difference between the two then please explain it.
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There are three ways of saying essentially the same thing:
(Notice that I converted your relative clause into an attributive adjective, just for simplicity.)
In each case, your purpose is to tell us something about lemons generally, not about some particular lemons. You are talking about the type of fruit. This is called a general statement. All three types of sentences can be used to convey general statements (see here for a reference to Alexander's Longman English Grammar).
The first uses a plural (also known as zero article), the second an indefinite article, and the third a definite article. All are usually acceptable.
In your case, the awkwardness of (1)—insofar as there is some—is caused by the word "fruit". Typically, this sentence would be rendered as something like: